Evidence for a functional membrane barrier in the transition zone between the flagellum and cell body of Chlamydomonas eugametos gametes

The molecular architecture of the ciliary transition zones

Cilia and flagella are specialized eukaryotic organelles projecting from the surface of eukaryotic cells that play a central role in various physiological processes, including cell motility, sensory perception, and signal transduction. At the base of these structures lies the ciliary transition zone, a pivotal region that functions as a gatekeeper and communication hub for ciliary activities. Despite its crucial role, the intricacies of its architecture remain poorly understood, especially given the variations in its organization across different cell types and species. In this review, we explore the molecular architecture of the ciliary transition zone, with a particular focus on recent findings obtained using cryotomography and super-resolution imaging techniques.

Composition, organization and mechanisms of the transition zone, a gate for the cilium

The cilium evolved to provide the ancestral eukaryote with the ability to move and sense its environment. Acquiring these functions required the compartmentalization of a dynein-based motility apparatus and signaling proteins within a discrete subcellular organelle contiguous with the cytosol. Here, we explore the potential molecular mechanisms for how the proximal-most region of the cilium, termed transition zone (TZ), acts as a diffusion barrier for both membrane and soluble proteins and helps to ensure ciliary autonomy and homeostasis. These include a unique complement and spatial organization of proteins that span from the microtubule-based axoneme to the ciliary membrane; a protein picket fence; a specialized lipid microdomain; differential membrane curvature and thickness; and lastly, a size-selective molecular sieve. In addition, the TZ must be permissive for, and functionally integrates with, ciliary trafficking systems (including intraflagellar transport) that cross the barrier and make the ciliary compartment dynamic. The quest to understand the TZ continues and promises to not only illuminate essential aspects of human cell signaling, physiology, and development, but also to unravel how TZ dysfunction contributes to ciliopathies that affect multiple organ systems, including eyes, kidney, and brain.

A cytoplasmic protein kinase couples engagement of Chlamydomonas ciliary receptors to cAMP-dependent cellular responses

The primary cilium is a cellular compartment specialized for receipt of extracellular signals essential for development and homeostasis. Although intraciliary responses to engagement of ciliary receptors are well studied, fundamental questions remain about the mechanisms and molecules that transduce ciliary signals into responses in the cytoplasm. During fertilization in the bi-ciliated alga Chlamydomonas reinhardtii, ciliary adhesion between plus and minus gametes triggers an immediate ∼10-fold increase in cellular cAMP and consequent responses in the cytoplasm required for cell-cell fusion. Here, we identify a new participant in ciliary signaling, Gamete-Specific Protein Kinase (GSPK). GSPK is essential for the adhesion-induced cAMP increase and for rapid gamete fusion. The protein is in the cytoplasm and the entire cellular complement responds to a signal from the cilium by becoming phosphorylated within 1 minute after ciliary receptor engagement. Unlike all other cytoplasmic events in ciliary signaling, GSPK phosphorylation is not responsive to exogenously added cAMP. Thus, during ciliary signaling in Chlamydomonas, a cytoplasmic protein is required to rapidly interpret a still uncharacterized ciliary signal to generate a cytoplasmic response.

How the Ciliary Membrane Is Organized Inside-Out to Communicate Outside-In

Cilia, organelles that move to execute functions like fertilization and signal to execute functions like photoreception and embryonic patterning, are composed of a core of nine-fold doublet microtubules overlain by a membrane. Distinct types of cilia display distinct membrane morphologies, ranging from simple domed cylinders to the highly ornate invaginations and membrane disks of photoreceptor outer segments. Critical for the ability of cilia to signal, both the protein and the lipid compositions of ciliary membranes are different from those of other cellular membranes. This specialization presents a unique challenge for the cell as, unlike membrane-bounded organelles, the ciliary membrane is contiguous with the surrounding plasma membrane. This distinct ciliary membrane is generated in concert with multiple membrane remodeling events that comprise the process of ciliogenesis. Once the cilium is formed, control of ciliary membrane composition relies on discrete molecular machines, including a barrier to membrane proteins entering the cilium at a specialized region of the base of the cilium called the transition zone and a trafficking adaptor that controls G protein-coupled receptor (GPCR) localization to the cilium called the BBSome. The ciliary membrane can be further remodeled by the removal of membrane proteins by the release of ciliary extracellular vesicles that may function in intercellular communication, removal of unneeded proteins or ciliary disassembly. Here, we review the structures and transport mechanisms that control ciliary membrane composition, and discuss how membrane specialization enables the cilium to function as the antenna of the cell.

Transient Internalization and Microtubule-Dependent Trafficking of a Ciliary Signaling Receptor from the Plasma Membrane to the Cilium

Cilia are ancient organelles used by unicellular and multicellular organisms not only for motility but also to receive and respond to multiple environmental cues, including light, odorants, morphogens, growth factors, and contact with cilia of other cells. Much is known about the cellular mechanisms that deliver membrane proteins to cilia during ciliogenesis. Execution of a ciliary signaling pathway, however, can critically depend on rapid alterations in the receptor composition of the cilium itself, and our understanding of the mechanisms that underlie these rapid, regulated alterations remains limited [1-6]. In the bi-ciliated, unicellular alga Chlamydomonas reinhardtii, interactions between cilia of mating type plus and mating type minus gametes mediated by adhesion receptors SAG1 and SAD1 activate a ciliary signaling pathway [7]. In response, a large, inactive pool of SAG1 on the plasma membrane of plus gametes rapidly becomes enriched in the peri-ciliary membrane and enters the cilia to become active and maintain and enhance ciliary adhesion and signaling [8-14]. Ciliary entry per se of SAG1 is independent of anterograde intraflagellar transport (IFT) [13], but the rapid apical enrichment requires cytoplasmic microtubules and the retrograde IFT motor, dynein 1b [14]. Whether the receptors move laterally within the plasma membrane or transit internally during redistribution is unknown. Here, in coupled immunolocalization/biochemical studies on SAG1, we show that, within minutes after gamete activation is initiated, cell-surface SAG1 is internalized, associates with an apico-basally polarized array of cytoplasmic microtubules, and returns to the cell surface at a peri-ciliary staging area for entry into cilia.

Chlamydomonas Basal Bodies as Flagella Organizing Centers

During ciliogenesis, centrioles convert to membrane-docked basal bodies, which initiate the formation of cilia/flagella and template the nine doublet microtubules of the flagellar axoneme. The discovery that many human diseases and developmental disorders result from defects in flagella has fueled a strong interest in the analysis of flagellar assembly. Here, we will review the structure, function, and development of basal bodies in the unicellular green alga Chlamydomonas reinhardtii, a widely used model for the analysis of basal bodies and flagella. Intraflagellar transport (IFT), a flagella-specific protein shuttle critical for ciliogenesis, was first described in C. reinhardtii. A focus of this review will be on the role of the basal bodies in organizing the IFT machinery.

The Bardet-Biedl syndrome protein complex is an adapter expanding the cargo range of intraflagellar transport trains for ciliary export

Bardet-Biedl syndrome (BBS) is a ciliopathy resulting from defects in the BBSome, a conserved protein complex. BBSome mutations affect ciliary membrane composition, impairing cilia-based signaling. The mechanism by which the BBSome regulates ciliary membrane content remains unknown. Chlamydomonas bbs mutants lack phototaxis and accumulate phospholipase D (PLD) in the ciliary membrane. Single particle imaging revealed that PLD comigrates with BBS4 by intraflagellar transport (IFT) while IFT of PLD is abolished in bbs mutants. BBSome deficiency did not alter the rate of PLD entry into cilia. Membrane association and the N-terminal 58 residues of PLD are sufficient and necessary for BBSome-dependent transport and ciliary export. The replacement of PLD's ciliary export sequence (CES) caused PLD to accumulate in cilia of cells with intact BBSomes and IFT. The buildup of PLD inside cilia impaired phototaxis, revealing that PLD is a negative regulator of phototactic behavior. We conclude that the BBSome is a cargo adapter ensuring ciliary export of PLD on IFT trains to regulate phototaxis.

Permeability barriers for generating a unique ciliary protein and lipid composition

Cilia (and flagella) are microtubule-based protrusions that are found in single or multiple copies on the surface of most eukaryotic cells. Defects in cilia formation and/or function have now been correlated with an expanding spectrum of human genetic diseases termed ciliopathies. Recent work indicates that cilia are indeed a bona fide organelle with a unique protein and lipid content that enables specific cellular functions. Despite the physiological and clinical relevance of cilia, our understanding of how a unique protein and lipid composition is generated for this organelle remains poor. Here we review recent work on the mechanisms that determine the protein and lipid content, and thus the functional outputs, of this unique organelle.

MKS5 and CEP290 Dependent Assembly Pathway of the Ciliary Transition Zone

Cilia have a unique diffusion barrier (“gate”) within their proximal region, termed transition zone (TZ), that compartmentalises signalling proteins within the organelle. The TZ is known to harbour two functional modules/complexes (Meckel syndrome [MKS] and Nephronophthisis [NPHP]) defined by genetic interaction, interdependent protein localisation (hierarchy), and proteomic studies. However, the composition and molecular organisation of these modules and their links to human ciliary disease are not completely understood. Here, we reveal Caenorhabditis elegans CEP-290 (mammalian Cep290/Mks4/Nphp6 orthologue) as a central assembly factor that is specific for established MKS module components and depends on the coiled coil region of MKS-5 (Rpgrip1L/Rpgrip1) for TZ localisation. Consistent with a critical role in ciliary gate function, CEP-290 prevents inappropriate entry of membrane-associated proteins into cilia and keeps ARL-13 (Arl13b) from leaking out of cilia via the TZ. We identify a novel MKS module component, TMEM-218 (Tmem218), that requires CEP-290 and other MKS module components for TZ localisation and functions together with the NPHP module to facilitate ciliogenesis. We show that TZ localisation of TMEM-138 (Tmem138) and CDKL-1 (Cdkl1/Cdkl2/Cdkl3/Cdlk4 related), not previously linked to a specific TZ module, similarly depends on CEP-290; surprisingly, neither TMEM-138 or CDKL-1 exhibit interdependent localisation or genetic interactions with core MKS or NPHP module components, suggesting they are part of a distinct, CEP-290-associated module. Lastly, we show that families presenting with Oral-Facial-Digital syndrome type 6 (OFD6) have likely pathogenic mutations in CEP-290-dependent TZ proteins, namely Tmem17, Tmem138, and Tmem231. Notably, patient fibroblasts harbouring mutated Tmem17, a protein not yet ciliopathy-associated, display ciliogenesis defects. Together, our findings expand the repertoire of MKS module-associated proteins—including the previously uncharacterised mammalian Tmem80—and suggest an MKS-5 and CEP-290-dependent assembly pathway for building a functional TZ.

The transition zone is a barrier structure required to maintain the dynamic composition and functional integrity of the cilium. This study describes the pathway by which the transition zone is assembled during cilium formation.

The primary cilium is a structure found in most animal cell types. Much like an antenna, it is responsible for sensing extracellular signals, including light and small molecules, and conveying this information to the receiving cell and respective tissue or organ. At the base of the cilium is the transition zone (TZ), which acts as a “gate” to regulate the entry and exit of ciliary proteins required for signal transduction. Here, we use the nematode Caenorhabditis elegans as a model system to dissect how different proteins within the TZ assemble to form a functional barrier. We find that the TZ protein MKS-5 (Rpgrip1/Rpgrip1L orthologue) forms the foundation for two different assembly pathways involving two distinct modules: Nephronophthisis (NPHP) and Meckel syndrome (MKS). We show that at the base of the MKS module is CEP-290, another TZ protein that assembles MKS module proteins, including a novel TZ protein we identify as TMEM-218. CEP-290 also helps assemble a potentially separate submodule containing TMEM-138 and CDKL-1. Notably, we provide evidence that the MKS module protein TMEM-17 facilitates cilium formation and is disrupted in the human disorder (ciliopathy) Oral-Facial-Digital Syndrome type 6 (OFD6). Together, our findings provide essential insights into the assembly pathway of the ciliary TZ and suggest further connections between the transition zone and human health.

Cby1 promotes Ahi1 recruitment to a ring-shaped domain at the centriole-cilium interface and facilitates proper cilium formation and function

Cby1 localizes to centrioles and antagonizes canonical Wnt signaling. Cby1^−/− mice have cystic kidneys, and Cby1 facilitates primary cilium formation and ciliary recruitment of Arl13b. Cby1 localizes to a distal centriolar domain with Ofd1 and Ahi1, and the amount of Ahi1 at the transition zone is reduced in Cby1^−/− cells.

Defects in centrosome and cilium function are associated with phenotypically related syndromes called ciliopathies. Cby1, the mammalian orthologue of the Drosophila Chibby protein, localizes to mature centrioles, is important for ciliogenesis in multiciliated airway epithelia in mice, and antagonizes canonical Wnt signaling via direct regulation of β-catenin. We report that deletion of the mouse Cby1 gene results in cystic kidneys, a phenotype common to ciliopathies, and that Cby1 facilitates the formation of primary cilia and ciliary recruitment of the Joubert syndrome protein Arl13b. Localization of Cby1 to the distal end of mature centrioles depends on the centriole protein Ofd1. Superresolution microscopy using both three-dimensional SIM and STED reveals that Cby1 localizes to an ∼250-nm ring at the distal end of the mature centriole, in close proximity to Ofd1 and Ahi1, a component of the transition zone between centriole and cilium. The amount of centriole-localized Ahi1, but not Ofd1, is reduced in Cby1^−/− cells. This suggests that Cby1 is required for efficient recruitment of Ahi1, providing a possible molecular mechanism for the ciliogenesis defect in Cby1^−/− cells.

Regulated membrane protein entry into flagella is facilitated by cytoplasmic microtubules and does not require IFT

The membrane protein composition of the primary cilium, a key sensory organelle, is dynamically regulated during cilium-generated signaling [1, 2]. During ciliogenesis, ciliary membrane proteins, along with structural and signaling proteins, are carried through the multicomponent, intensely studied ciliary diffusion barrier at the base of the organelle [3-8] by intraflagellar transport (IFT) [9-18]. A favored model is that signaling-triggered accumulation of previously excluded membrane proteins in fully formed cilia [19-21] also requires IFT, but direct evidence is lacking. Here, in studies of regulated entry of a membrane protein into the flagellum of Chlamydomonas, we show that cells use an IFT-independent mechanism to breach the diffusion barrier at the flagellar base. In resting cells, a flagellar signaling component [22], the integral membrane polypeptide SAG1-C65, is uniformly distributed over the plasma membrane and excluded from the flagellar membrane. Flagellar adhesion-induced signaling triggers rapid, striking redistribution of the protein to the apical ends of the cells concomitantly with entry into the flagella. Protein polarization and flagellar enrichment are facilitated by cytoplasmic microtubules. Using a conditional anterograde IFT mutant, we demonstrate that the IFT machinery is not required for regulated SAG1-C65 entry into flagella. Thus, integral membrane proteins can negotiate passage through the ciliary diffusion barrier without the need for a motor.

Transport and localization of signaling proteins in ciliated cells

Most cells in the human body elaborate cilia which serve a wide variety of functions, including cell and tissue differentiation during development, sensing physical and chemical properties of the extracellular milieu and mechanical force generation. Common among cilia is the transduction of external stimuli into signals that regulate the activities of the cilia and the cells that possess them. These functions require the transport and localization of specialized proteins to the cilium, a process that many recent studies have shown to be vital for normal cell function and, ultimately, the health of the organism. Here we discuss several mechanisms proposed for the transport and localization of soluble and peripheral membrane proteins to, or their exclusion from the ciliary compartment with a focus on how the structure of the cytoplasm and the size and shape of proteins influence these processes. Additionally, we examine the impact of cell and protein structure on our ability to accurately measure the relative concentrations of fluorescently tagged proteins amongst various cellular domains, which is integral to our understanding of the molecular mechanisms underlying protein localization and transport.

[The rebirth of the ultrastructure of cilia and flagella]

The sensory and motility functions of eukaryotic cilia and flagella are essential for cell survival in protozoans and for cell differentiation and homoeostasis in metazoans. Ciliary biology has benefited early on from the input of electron microscopy. Over the last decade, the visualization of cellular structures has greatly progressed, thus it becomes timely to review the ultrastructure of cilia and flagella. Briefly touching upon the typical features of a 9+2 axoneme, we dwell extensively on the transition zone, the singlet zone, the ciliary necklace, cap and crown. The relation of the singlet zone to sensory and/or motile function, the link of the ciliary cap to microtubule dynamics and to ciliary beat, the involvement of the ciliary crown in ovocyte and mucosal propulsion, and the role of the transition zone/the ciliary necklace in axonemal stabilization, autotomy and as a diffusion barrier will all be discussed.

Ultrastructure of cilia and flagella - back to the future!

Eukaryotic cilia and flagella perform motility and sensory functions which are essential for cell survival in protozoans, and to organism development and homoeostasis in metazoans. Their ultrastructure has been studied from the early beginnings of electron microscopy, and these studies continue to contribute to much of our understanding about ciliary biology. In the light of the progress made in the visualization of cellular structures over the last decade, we revisit the ultrastructure of cilia and flagella. We briefly describe the typical features of a 9+2 axoneme before focusing extensively on the transition zone, the ciliary necklace, the singlet zone, the ciliary cap and the ciliary crown. We discuss how the singlet zone is linked to sensory and/or motile function, the contribution of the ciliary crown to ovocyte and mucosal propulsion, and the relationship between the ciliary cap and microtubule growth and shortening, and its relation to ciliary beat. We further examine the involvement of the transition zone/the ciliary necklace in axonemal stabilization, autotomy and as a diffusion barrier.

Ciliary diffusion barrier: the gatekeeper for the primary cilium compartment

The primary cilium is a cellular antenna that detects and transmits chemical and mechanical cues in the environment through receptors and downstream signal proteins enriched along the ciliary membrane. While it is known that ciliary membrane proteins enter the cilium by way of vesicular and intraflagellar transport, less is known about how ciliary membrane proteins are retained in, and how apical membrane proteins are excluded from the cilium. Here, we review evidence for a membrane diffusion barrier at the base of the primary cilium, and highlight the recent finding of a septin cytoskeleton diffusion barrier. We also discuss candidate ciliopathy genes that may be involved in formation of the barrier, and the role of a diffusion barrier as a common mechanism for compartmentalizing membranes and lipid domains.

Trafficking to the ciliary membrane: how to get across the periciliary diffusion barrier?

The primary cilium organizes numerous signal transduction cascades, and an understanding of signaling receptor trafficking to cilia is now emerging. A defining feature of cilia is the periciliary diffusion barrier that separates the ciliary and plasma membranes. Although lateral transport through this barrier may take place, polarized exocytosis to the base of the cilium has been the prevailing model for delivering membrane proteins to cilia. Key players for this polarized exocytosis model include the GTPases Rab8 and Rab11, the exocyst, and possibly the intraflagellar tranport machinery. In turn, the sorting of membrane proteins to cilia critically relies on the recognition of ciliary targeting signals by sorting machines such as the BBSome coat complex or the GTPase Arf4. Finally, some proteins need to exit from cilia, and ubiquitination may regulate this step. The stage is now set to dissect the interplay between signaling and regulated trafficking to and from cilia.

A septin diffusion barrier at the base of the primary cilium maintains ciliary membrane protein distribution

In animal cells, the primary cilium transduces extracellular signals through signaling receptors localized in the ciliary membrane, but how these ciliary membrane proteins are retained in the cilium is unknown. We found that ciliary membrane proteins were highly mobile, but their diffusion was impeded at the base of the cilium by a diffusion barrier. Septin 2 (SEPT2), a member of the septin family of guanosine triphosphatases that form a diffusion barrier in budding yeast, localized at the base of the ciliary membrane. SEPT2 depletion resulted in loss of ciliary membrane protein localization and Sonic hedgehog signal transduction, and inhibited ciliogenesis. Thus, SEPT2 is part of a diffusion barrier at the base of the ciliary membrane and is essential for retaining receptor-signaling pathways in the primary cilium.

The ciliary membrane

Cilia and flagella are important organizing centers for signaling in both development and disease. A key to their function is a poorly characterized barrier at their base that allows the protein and lipid composition of the ciliary membrane to be distinct from that of the plasma membrane. We review current models of ciliary membrane biogenesis, highlighting several structures, including the ciliary necklace and ciliary pocket, that appear during biogenesis and that likely contribute to the barrier. The regulated movement of membrane proteins and lipids across this barrier is central to the sensory function of these organelles.

Lateral transport of Smoothened from the plasma membrane to the membrane of the cilium

A distinct pathway for ligand-stimulated lateral trafficking of the hedgehog protein Smoothened into cilia is described.

The function of primary cilia depends critically on the localization of specific proteins in the ciliary membrane. A major challenge in the field is to understand protein trafficking to cilia. The Hedgehog (Hh) pathway protein Smoothened (Smo), a 7-pass transmembrane protein, moves to cilia when a ligand is received. Using microscopy-based pulse-chase analysis, we find that Smo moves through a lateral transport pathway from the plasma membrane to the ciliary membrane. Lateral movement, either via diffusion or active transport, is quite distinct from currently studied pathways of ciliary protein transport in mammals, which emphasize directed trafficking of Golgi-derived vesicles to the base of the cilium. We anticipate that this alternative route will be used by other signaling proteins that function at cilia. The path taken by Smo may allow novel strategies for modulation of Hh signaling in cancer and regeneration.

Targeting proteins to the ciliary membrane

Most vertebrate cell types display solitary nonmotile cilia on their surface that serve as cellular antennae to sense the extracellular environment. These organelles play key roles in the development of mammals by coordinating the actions of a single cell with events occurring around them. Severe defects in cilia lead to midgestational lethality in mice while more subtle defects lead to pathology in most organs of the body. These pathologies range from cystic diseases of the kidney, liver, and pancreas, to retinal degeneration, to bone and skeletal defects, hydrocephaly, and obesity. The sensory functions of cilia rely on proteins localized specifically to the ciliary membrane. Even though the ciliary membrane is a subdomain of the plasma membrane and is continuous with the plasma membrane, cells have the ability to specifically localize proteins to this domain. In this chapter, we will review what is currently known about the structure and function of the ciliary membrane. We will further discuss ongoing work to understand how the ciliary membrane is assembled and maintained, and discuss protein machinery that is thought to play a role in sorting or trafficking proteins to the ciliary membrane.

Regulated targeting of a protein kinase into an intact flagellum. An aurora/Ipl1p-like protein kinase translocates from the cell body into the flagella during gamete activation in chlamydomonas

In the green alga Chlamydomonas reinhardtii flagellar adhesion between gametes of opposite mating types leads to rapid cellular changes, events collectively termed gamete activation, that prepare the gametes for cell-cell fusion. As is true for gametes of most organisms, the cellular and molecular mechanisms that underlie gamete activation are poorly understood. Here we report on the regulated movement of a newly identified protein kinase, Chlamydomonas aurora/Ipl1p-like protein kinase (CALK), from the cell body to the flagella during gamete activation. CALK encodes a protein of 769 amino acids and is the newest member of the aurora/Ipl1p protein kinase family. Immunoblotting with an anti-CALK antibody showed that CALK was present as a 78/80-kDa doublet in vegetative cells and unactivated gametes of both mating types and was localized primarily in cell bodies. In cells undergoing fertilization, the 78-kDa CALK was rapidly targeted to the flagella, and within 5 min after mixing gametes of opposite mating types, the level of CALK in the flagella began to approach levels normally found in the cell body. Protein synthesis was not required for targeting, indicating that the translocated CALK and the cellular molecules required for its movement are present in unactivated gametes. CALK was also translocated to the flagella during flagellar adhesion of nonfusing mutant gametes, demonstrating that cell fusion was not required for movement. Finally, the requirement for flagellar adhesion could be bypassed; incubation of cells of a single mating type in dibutyryl cAMP led to CALK translocation to flagella in gametes but not vegetative cells. These experiments document a new event in gamete activation in Chlamydomonas and reveal the existence of a mechanism for regulated translocation of molecules into an intact flagellum.

A 210 kDa protein is located in a membrane-microtubule linker at the distal end of mature and nascent basal bodies

A monoclonal antibody raised against purified flagellar basal apparatuses from the green flagellate Spermatozopsis similis reacted with a protein of 210 kDa (p210) in western blots. The protein was partially cloned by immunoscreening of a cDNA library. The sequence encoded a novel protein rich in alanine (25%) and proline (20%), which contained regions similar to proteins of comparable amino acid composition such as extracellular matrix components or the membrane-cytoskeletal linker synapsin. Using a polyclonal antibody (anti-p210) raised against the C-terminal part of p210, it was shown that the protein was highly enriched in the basal apparatuses. Immunogold electron microscopy of isolated cytoskeletons or whole cells revealed that p210 was located in the flagellar transition region. The protein was part of the Y-shaped fibrous linkers between the doublet microtubules and the flagellar membrane, as indicated by statistical analysis of post-labeled sections using anti-centrin and anti-tubulin as controls. In premitotic cells p210 was located in a fibrous layer at the distal end of nascent basal bodies, which was perforated by the outgrowing axoneme. During deflagellation the protein remained at the basal body but we observed changes in its distribution, indicating that p210 partially moved to the tip of the basal body. p210 can be used as a marker to determine basal body position, orientation (parallel or antiparallel) and number in S. similis by indirect immunofluorescence. We suppose that p210 is involved in linking basal bodies to the plasma membrane, which is an important step during ciliogenesis.

The role of preassembled cytoplasmic complexes in assembly of flagellar dynein subunits

Previous work has revealed a cytoplasmic pool of flagellar precursor proteins capable of contributing to the assembly of new flagella, but how and where these components assemble is unknown. We tested Chlamydomonas outer-dynein arm subunit stability and assembly in the cytoplasm of wild-type cells and 11 outer dynein arm assembly mutant strains (oda1-oda11) by Western blotting of cytoplasmic extracts, or immunoprecipitates from these extracts, with five outer-row dynein subunit-specific antibodies. Western blots reveal that at least three oda mutants (oda6, oda7, and oda9) alter the level of a subunit that is not the mutant gene product. Immunoprecipitation shows that large preassembled flagellar complexes containing all five tested subunits (three heavy chains and two intermediate chains) exist within wild-type cytoplasm. When the preassembly of these subunits was examined in oda strains, we observed three patterns: complete coassembly (oda 1, 3, 5, 8, and 10), partial coassembly (oda7 and oda11), and no coassembly (oda2, 6, and 9) of the four tested subunits with HCbeta. Our data, together with previous studies, suggest that flagellar outer-dynein arms preassemble into a complete Mr approximately 2 x 10(6) dynein arm that resides in a cytoplasmic precursor pool before transport into the flagellar compartment.

Brefeldin A affects synthesis and integrity of a eukaryotic flagellum

Eukaryotic flagella and cilia are highly dynamic organelles. In green algae like Chlamydomonas reinhardtii, flagella absorption and resynthesis is a normal process during the vegetative cell cycle. Rapid regeneration also occurs after stress-induced shedding of flagella. Ca2+ ions, protein synthesis, and a kinase activity are the main factors known to affect resynthesis. Recently, we have detected that certain small G proteins (Ypt/Rab) and a GTPase regulator (GDP dissociation inhibitor), known as regulatory elements of intracellular vesicle transport, are present in flagellar membranes of green algae, raising the possibility that the organelle's synthesis and/or integrity depends on functional membrane traffic. In this study, we examined the effect of brefeldin A (BFA), an inhibitor of intracellular membrane flow and Golgi function in animal and plant cells, on flagella regeneration in the colonial green alga Gonium pectorale. We show that low BFA concentrations (< 1 microgram/ml) inhibit flagella out-growth, while higher amounts cause dose-dependent deflagellation and cell death. Our findings provide experimental evidence for a direct connection between intracellular transport and eukaryotic flagella synthesis.

Electrodynamics of microtubular motors: the building blocks of a new model

Microtubules are ubiquitous components of the cytoskeleton. They participate in many motility processes ranging from intracellular transport or chromosome movement during mitosis to ciliary and flagellar beating. The biophysical mechanism inherent in the generation and control of movement in all these motility phenomena has not yet been entirely elucidated. The authors propose a new model based on a charge transfer mechanism capable of shedding a new light on the molecular foundations of all motility processes. Electron transfer along the microtubular lattice is responsible for activation and control of all microtubule-associated ATPases (i.e. force generating enzymes). Microtubules are thus shown to be the basic motors of cell dynamics. The model is first applied to intracellular transport and ciliary and flagellar beating. Through two additional examples, the authors show the heuristic capabilities of the suggested hypothesis. The application of charge transfer control to the Protozoan Euglena gracilis leads to a plausible model capable of accounting for its phototactic response mechanism. Furthermore, the model allows a new interpretation of the electrophysiological response in vertebrate photoreceptors.

Compartmentalization, processing and redistribution of the plasma membrane protein CE9 on rodent spermatozoa. Relationship of the annulus to domain boundaries in the plasma membrane of the tail

Western blotting, immunofluorescence and immunogold electron microscopy were used to examine the compartmentalization, processing and redistribution of the integral plasma membrane protein CE9 on the spermatozoa of rats, mice and hamsters. In each species examined, spermatozoal CE9 was found to undergo endoproteolytic processing followed by a net redistribution from the posterior-tail domain into the anterior-tail domain of the plasma membrane during epididymal maturation. Compared to spermatozoa of the rat and mouse, those of the hamster were found to express a greater proportion of their CE9 within the anterior-tail plasma membrane domain at all stages of maturation. As a consequence, CE9 was judged to be a suitable marker for two different spermatozoal plasma membrane domains: the posterior-tail plasma membrane domain (spermatozoa from the testis and caput epididymidis of the rat and mouse) and the anterior-tail domain (spermatozoa from the cauda epididymidis of the hamster). Immunogold electron microscopy was used to pinpoint the positions of the boundaries of these CE9-containing plasma membrane domains at a high level of resolution. In each case, the position of the CE9 domain boundary was found to be strongly correlated with that of the subplasmalemmal electron-dense ring known as the annulus. The precise spatial relationship between the CE9 domain boundary and the annulus was, however, found to differ significantly among species and/or as a function of maturation.

Polyphosphoinositol lipids in Chlamydomonas eugametos gametes

In Chlamydomonas eugametos gametes, phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) comprised 0.4 and 0.3% of the whole-cell phospholipids. They were concentrated in the plasma membrane around the cell body and were present in low concentrations in the flagellar membrane. When gametes were fed (32)PO 4 (-) , the label was rapidly incorporated into PtdInsP and PtdInsP2 and only slowly incorporated into structural lipids such as phosphatidylethanolamine and phosphatidylglycerol. Similarly, when a pulse of (32)PO 4 (-) was chased with PO 4 (-) , the label was rapidly lost from the polyphosphoinositol lipids but not from the structural lipids. The major fatty acids in the polyphosphoinositides were C-22 carbon polyenoic acids (70%). The significance of these results in relationship to intracellular signalling via inositol phosphates and Ca(2+) is discussed.

Filipin-sterol complexes in molluscan gill ciliated epithelial cell membranes: intercalation into ciliary necklaces and induction of gap junctional particle arrays

Freeze-fracture electron microscopy has been used in conjunction with the antibiotic filipin to investigate possible differences in the distribution of sterols in ciliary and somatic cell membranes of scallop and mussel gill epithelial cells. Contrary to previous reports, we find that filipin-sterol lesions can occur among the strands of the ciliary necklace but they are partially excluded from the smooth neck region above the necklace where the membrane is tightly apposed to the axonemal microtubules. No obvious differences in filipin-sterol lesions occur in the membranes of mussel gill cilia of varying mechanical sensitivity. Although abundant in the apical plasma membrane, filipin-sterol complexes are rare within the membranes of microvilli. Filipin-sterol lesions form outside the loosely parallel particle strands of septate junctions, sometimes increasing their relative orderliness. At sufficiently high density, filipin-sterol protrusions within the plasma membrane result in mass aggregation of gap junctions, possibly through recruitment of unorganized connexons.

Monoclonal antibodies to sperm surface antigens of the brown alga Fucus serratus exhibit region-, gamete-, species- and genus-preferential binding

A panel of twelve monoclonal antibodies (MAbs), designated FS1 to FS12, have been raised against surface antigens of Fucus serratus sperm. The antibodies were selected on the basis that they show region-, gamete-, species- or genus-preferential binding. Indirect immunofluorescence shows that the antigens bound by the MAbs are distributed non-randomly over the cell surface. Seven MAbs (FS1, FS3, FS4, FS6, FS8, FS9, FS10) bind antigens located primarily on the cell body, while the others (FS2, FS5, FS7, FS11, FS12) bind antigens located primarily on the anterior flagellum. Of the MAbs that label the anterior flagellum, FS2, FS5, FS7 and FS12 form a 'halo' at the perimeter of the flagellum. Electron microscopic-immunogold studies indicate that the 'halo' results from labelling of the mastigonemes, as opposed to the flagellar plasmamembrane. Gamete-preferential binding of antibodies was detected using an enzyme-linked immunosorbent assay with egg membrane vesicles. Eight of the MAbs bind sperm antigens not common to eggs, though FS2, FS4, FS5 and FS9 bind antigens present on both sperm and eggs. In studies of species- and genus-specificity FS2, FS3, FS5, FS6, FS7, FS8, FS10, FS11 and FS12 exhibit genus-preferential binding, labelling sperm of F. serratus and F. vesiculosus more intensely than that of Ascophyllum nodosum. Only FS10 showed marked species-preferential binding, labelling sperm of F. serratus much more intensely than that of F. vesiculosus.

Use of carbohydrate probes in conjunction with fluorescence-activated cell sorting to select mutant cell lines of Chlamydomonas with defects in cell surface glycoproteins

Two carbohydrate-binding probes (the lectin concanavalin A and the anti-carbohydrate monoclonal antibody FMG-1) have been utilized in conjunction with fluorescence-activated cell sorting to select cell lines of Chlamydomonas reinhardtii that contain defects in cell surface-exposed glycoproteins. Two very different selection strategies (sorting cells with the lowest binding for the FMG-1 monoclonal antibody or the highest binding of concanavalin A) yield a class of mutant cells that exhibit a total lack of binding of the monoclonal antibody to cell wall and plasma membrane glycoproteins along with an increased affinity for concanavalin A. Detailed characterization of one such mutant cell line, designated L-23, is provided. The subtle glycosylation defect exhibited by this cell line does not alter the ability of the affected glycoproteins to be targeted to the flagellar membrane and does not affect the expression of flagellar surface motility, a phenomenon that appears to involve the major concanavalin A-binding glycoprotein of the flagellar membrane. This approach has general applicability for dissecting the role of carbohydrate epitopes in the targeting and function of any cell surface glycoprotein for which suitable carbohydrate probes are available.

Monoclonal antibodies to surface glycoconjugates in Chlamydomonas eugametos recognize strain-specific O-methyl sugars

Monoclonal antibodies are described that are directed against cell surface components of the unicellular green alga Chlamydomonas eugametos. These antibodies recognize strain-specific epitopes which occur at the surface of vegetative and gametic cells. Two different groups of epitopes are distinguished that are never detectable together in one clonal cell culture. Evidence is presented showing that the antigenicity of cell surface molecules is a consequence of the presence of particular O-methylated sugars. Monoclonal antibodies reacting with one group of epitopes were studied in more detail, and immunoprecipitation and Western-blot studies showed that these epitopes can be arranged into four classes. The use of these monoclonal antibodies as strain-specific markers in light- and electron-microscopical techniques is illustrated.

Light dependence of sexual agglutinability in Chlamydomonas eugametos

The mating activity of mating-type plus gametes of Chlamydomonas eugametos depends on light. Cells lost their ability to agglutinate with mating-type minus gametes after a dark period of 30 min. They regained their agglutinability after 10 min exposure to light. Other mating reactions, such as tipping and flagellar tip activation, were not dependent upon light. Since cycloheximide and tunicamycin did not affect the light-induced activation of flagellar agglutinability, no protein synthesis or glycosylation is involved in this process. Equal amounts of biologically active agglutination factor could be extracted from cells placed either in light or in darkness. A minor portion of the active material was found to be located on the flagellar surface of illuminated cells. No active material was found on the flagellar surface of dark-exposed cells, whereas their cell bodies contained the same amount of active material as the cell bodies of illuminated cells. Since a light-induced flow of agglutination factors from the cell body to the flagella could not be detected and dark-exposed cells could be slightly activated by amputation or fixation by glutaraldehyde, we propose that light affects flagellar agglutinability by an in-situ modification of the agglutination factor on the flagella. When mt (+) and mt (-) strains were crossed and the progeny examined for light-sensitivity, it was apparent that this phenomenon is not mating type-linked.